To protect individuals from potentially dangerous airborne agents, a plurality of approaches have been taken over the years. For instance, woven and non-woven fibrous materials have been designed for formation of masks, clothing, and the like to filter airborne agents. In general, there are five modes of particle removal in such fibrous filters: sieve effect, inertial impaction, interception, Brownian motion, and electrostatic charge effect. While sieve effect, in which particles above a certain size are physically prevented passage through the filter media, is often a primary removal method found in liquid filtration, fibrous gas and air filters utilize this mode to a lesser degree. The primary particle removal methods of fibrous gas and air filters include inertial impaction, in which a relatively large particle collides with a fiber and adheres as its inertia prevents adjustment to the streamline flow around the fiber; interception, in which a particle that is following the gas streamline through the filter comes within one particle radius of the fiber and is captured thereon; Brownian motion, in which the random path due to collision between particles and carrier gas molecules leads to random motion of the particles that increase the probability of impaction or interception between the particle and a fiber; and electrostatic capture, in which an opposite electrostatic charge between the fiber and the particle leads to attraction and adherence.
Unfortunately, optimization of one parameter of fibrous filtration media generally leads to a decline in another parameter. For instance, increase in porosity in a filter material can lower pressure drop across the filter and increase holding capacity, but will decrease collection efficiency. Decrease in fiber size and the resulting increase in fiber density possible due to the smaller fiber size can improve collection efficiency, but can lead to an increase in pressure drop across the filter. Similarly, forming a thicker filter can improve holding capacity, but will increase the pressure drop across the filter. Electrostatic charge has been applied to synthetic fibers to improve electrostatic capture, but these fibers tend to be large diameter fibers, and large fibers carry a decreased probability of a particle colliding with a fiber.
Moreover, when considering utilization of the filter materials as protection devices for individuals, comfort of the wearer and time of effectiveness become important. For instance, a thick, heavy filtration material that provides high holding capacity can be unwearable due to weight, body heat build-up and the like. Additionally, high pressure drop across a filtration material can prevent air flow all together (in the case of clothing) or cause breathing difficulties (in the case of face masks). Moreover, a filter material that will reach holding capacity in very short order will be of little practical use.
In an attempt to overcome some of the problems presented by filter materials, attempts have been made to develop materials that include active chemistries. For instance, filter elements including decontamination media are often incorporated in protective masks that can remove and/or detoxify agents from air breathed by the wearer. Unfortunately, functionalization of filtration materials does not solve some problems of protective materials. For instance, it has proven very difficult to optimize contact time between functionalized filter material and the air to be filtered while maintaining limited pressure drop across the material, weight of the material and the related comfort of the wearer.
Accordingly, while the addition of active chemistries to fibrous filtration media has greatly improved the art, room for further improvement exists. What are needed in the art are fibrous materials and methods for forming such material that can provide good contact time between active chemistries and air to be filtered as well as provide good holding capacities for filtered agents, while minimizing pressure drop across the materials.